An online auction for the gun George Zimmerman used to kill Trayvon Martin has received bids of more than $25million (£17.3 million), although it is unclear if the bidders are serious.

The website for United Gun Group began hosting the auction on Thursday after another website, GunBroker.com , took it down saying it wanted "no part in the listing on our website or in any of the publicity it is receiving".

Hours later, United Gun Group tweeted that it would post Mr Zimmerman's ad. The new link was posted, along with a statement from Mr Zimmerman.

The site calls itself a "social market place for the firearms community".

Bidding on the 9 mm Kel-Tec PF-9 pistol began at 5,000 US dollars (£3,460). By this morning, the site showed bidding had surpassed $25million, with the leading bidder using the screen name "Racist McShootface".

The 9mm Kel-Tec PF-9 pistol (Image: Universal News & Sport)

Critics called the auction an insensitive move to profit from the killing.

Mr Zimmerman had told Florida TV station WOFL on Thursday that the pistol was returned to him by the US Justice Department, which took it after he was acquitted in Trayvon's 2012 shooting death.

The original listing said a portion of the proceeds would go towards fighting what Mr Zimmerman calls violence by the Black Lives Matter movement against law enforcement officers, combating anti-gun rhetoric of Democratic presidential candidate Hillary Clinton and ending the career of lawyer Angela Corey, who led his prosecution.

It ended with a Latin phrase that translates as "if you want peace, prepare for war".

The 32-year-old has said he was defending himself when he killed Trayvon, an unarmed black 17-year-old, in a gated community near Orlando. Trayvon, who lived in Miami with his mother, was visiting his father at the time.

Trayvon Martin was shot by Zimmerman

The case sparked protests and a national debate about race relations. The Justice Department later decided not to prosecute Mr Zimmerman on civil rights charges.

Lucy McBath, the mother of another black teenager shot by a white man during an argument at a Jacksonville convenience store in 2012, said the auction reflected a "deplorable lack of value for human life".

"I am deeply disappointed that the man who killed Trayvon Martin is trying to sell the very gun he used to cut that precious life short to raise money," she said in a written statement.

The killing of her son, 17-year-old Jordan Davis, by Michael Dunn drew parallels at the time to the Zimmerman-Trayvon case. Dunn told police he had felt threatened by Jordan. Unlike Mr Zimmerman, he was convicted of murder.

Since Mr Zimmerman was acquitted, he has been charged with assault based on complaints from two girlfriends.

Demonstrators march during a protest against the acquittal of George Zimmerman (Image: REUTERS)

Both women later refused to press charges and Mr Zimmerman was not prosecuted.

His estranged wife, Shellie Zimmerman, also accused him of smashing her iPad during an argument days after she filed divorce papers. No charges were filed because of lack of evidence. They were divorced in January.

Trayvon's parents declined to address Mr Zimmerman's actions in statements made through representatives.

His mother, Sybrina Fulton, said through a lawyer that she would rather focus on her work with the Trayvon Martin Foundation than respond to "Zimmerman's actions".